Final answer:
Conjugation of a C=O bond with a pi system causes a shift of the IR absorption peak to a lower frequency, typically between 1650-1700 cm⁻¹. This shift is due to electron delocalization, which stabilizes the molecule and reduces the carbonyl double bond's vibrational energy.
Step-by-step explanation:
The location of C=O absorption peaks in infrared (IR) spectroscopy can be influenced by the conjugation with other pi bonds. Normally, a C=O bond has an absorption peak at around 1700-1750 cm⁻¹. However, when the carbonyl group is conjugated with a pi system, such as in conjugated unsaturated ketones or amides, the absorption peak shifts to a lower frequency, ranging typically from 1650-1700 cm⁻¹. Conjugation lowers the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), leading to a reduction in the energy required for excitation, which translates to a peak at a longer wavelength or a lower frequency in the IR spectrum. This effect is observed because the presence of conjugation with the pi system leads to electron delocalization, which stabilizes the molecule and reduces the bond strength of the carbonyl group, thus lowering the frequency of the C=O stretching vibration.